Multimodal polyethylenes are widely used in the art in various applications, such as fibers, moldings, films and pipes, in view of the improved properties that they exhibit over monomodal polyethylenes. Multimodal polyethylenes present molecular weight distribution curves having more than one molecular weight peak, due to the presence of a plurality of polymer fractions having different molecular weights; in view of their broader molecular weight distribution, multimodal polyethylenes can also be processed more easily with respect to monomodal polyethylenes.
Various methods are known in the art to produce multimodal polyethylene, including post reactor melt blending, single stage processes carried out in the presence of a catalyst system comprising a mixture of different catalysts, and multistage processes. The method used determines the properties of the polyethylene, in that the properties of a multimodal polyethylene depend not only on the properties of the single polymer fractions thereof, but also by the quality of the mixing of these fractions. A poor mixing quality results, inter alia, in a low stress cracking resistance and adversely affects the creep behaviour of articles made of such polyethylenes.
Melt blending techniques, commonly carried out in an extruder, are expensive, cumbersome and time consuming; moreover, the mixing quality of the fractions is not sufficient for many applications.
Alternatively, multimodal polyethylene may be produced in a single reactor by using catalyst systems comprising at least two different ethylene polymerization catalysts, able to produce polyethylene fractions having different molecular weight.
Various families of polymerization catalysts are known in the art, such as Ziegler catalysts, metallocene catalysts and the more recently developed transition metal complexes comprising ligands other than cyclopentadienyl. For instance, WO 98/27124 discloses 2,6-bis(imino)pyridyl complexes of iron and cobalt as catalysts for homo- or co-polymerization of ethylene. J. Am. Chem. Soc. 127, 13019-13029 (2005) describes the preparation of several bis-iminopyridinato Ziegler catalysts and their activities in the polymerization of ethylene.
Catalyst mixtures comprising different active centres have been described in the prior art to prepare polymers having broader distributions in processes using a single reactor. For instance, the use of catalyst compositions comprising at least two different ethylene polymerization catalysts of the Ziegler type or the metallocene type is described for instance in WO 95/11264, which discloses the use of a combination of such catalysts to produce a polyethylene having a broad molecular weight distribution.
WO 99/46302 describes a catalyst composition for the polymerization of alpha-olefins comprising a 2,6-bis(imino)pyridyl iron catalyst and another catalyst, such as a zirconocene or a Ziegler catalyst; WO 05/103096 discloses a catalyst composition comprising a 2,6-bis(imino)pyridyl iron catalyst and a hafnocene catalyst.
The above documents describe the use of a mixed catalyst system in a single polymerization step. However, in the continuous polymerization of olefins using hybrid catalysts in a single reactor, there is the problem that the properties of the polymers obtained significantly on the ratio of the active centres present. Fluctuations in the composition of different batches of hybrid catalysts used can thus lead to different proportions of the polymer components formed by the individual catalyst components. Moreover, catalyst aging, in particular if one of the components is more sensitive than the other(s), can also result in different products even when the same batch is used. Fluctuations of the polymerization conditions can also influence the activity of the catalyst components used in different ways, so that different proportions of the polymer components formed by the individual catalyst components can also result. There is therefore a great need, in particular in the case of hybrid catalysts, for ways of controlling the composition of the polymers formed.
Multistage polymerization processes are normally carried out in at least two steps, which may be carried out in the same reactor or in at least two reactors operating in series; each step is conducted under different process conditions, in order to obtain polyethylene fractions having different molecular weights and/or different monomer compositions. The commonest way to obtain fractions of different molecular weight is to use different hydrogen concentrations in the reactors, while fractions of different compositions may be obtained by using different comonomer concentrations.
Such multimodal polyethylene blends are often produced using reactor cascades, i.e. two or more polymerization reactors connected in series, wherein the polymerization of the low molecular weight component occurs in the first reactor and the polymerization of the high molecular weight component occurs in the next reactor (see, for example, M. Rätzsch, W. Neiβl “Bimodale Polymerwerkstoffe auf der Basis von PP and PE” in “Aufbereiten von Polymeren mit neuartigen Eigenschaften” pp. 3-25, VDI-Verlag, Düsseldorf 1995).
A disadvantage of this process is that relatively large amounts of hydrogen have to be added to produce the fraction having the relatively lower molecular weight, and as a consequence, especially the low molecular weight polyethylene fraction has a very low content of vinyl groups, generally lower than 0.3. Moreover, when different hydrogen concentrations are used in the different reactors, it is technically difficult to prevent the hydrogen or any other molecular weight regulator added in the first reactor from getting into the second reactor.
The same problem is encountered when different comonomers, or different concentrations of comonomers are used in the various polymerization steps; in this case, a high outlay in terms of apparatus is necessary.
Various approaches have been tried in order to solve this problem. Thus, WO 00/50466 and WO 02/24768 describe polymerization processes using hybrid catalysts, in each of which two different hybrid catalysts are introduced into a reactor, with the two hybrid catalysts comprising the same catalyst components but in a different ratio. The ratio of the polymer components formed to one another can then be controlled by regulating the ratio of the two hybrid catalysts. However, to achieve this it is necessary to install two different metering systems on one reactor and regulate these relative to one another and also to produce and keep available two different catalyst solids for each polymer type produced.
It is therefore an object of the present invention to provide a multistage process able to overcome the above-mentioned problems.